In the chemical engineering arts, structured tower packings are a known class of devices used to effect heat and mass transfer between vapor and liquid streams in a tower for the purposes of distillation, rectification, fractionation, stripping, splitting, absorption, desorption, cooling, heating, and similar unit operations. Towers containing structured tower packing are a form of packed towers, and are most often operated with the vapor and liquid streams in counter-current flow.
The primary design object in a structured tower packing is to provide ample opportunity for the liquid and vapor which are typically flowing in counter-current relation through the tower to come into intimate and extended reactions with one another so that the mass and energy exchange reactions between the vapor and liquid may proceed. These reactions are in most instances gas film coefficient controlled, ultimately, and in this circumstance means that care must be taken to obtain good gas distribution, and such turbulence and mixing in the gas as can be readily had, so that the vapor or gas film at the interface is as thin as possible.
These reactions are also strongly dependent on the area of contact between the vapor and the liquid, and this circumstance means that care should be taken to obtain very good liquid distribution over the surface of the packing so that the area of contact is as large as can be obtained.
Structured tower packings are passive devices in the sense that they have no moving parts, and no external power is input directly to them. As a consequence, the objects of obtaining good vapor and liquid distribution and good intimate contact between the vapor and the liquid must be obtained, if at all, by configuring the structure of the packing and its surface and through-the-surface features to maximize the liquid and vapor distribution in a passive manner.
Within the field of structured tower packing, one type which has been of technical and commercial importance in recent times is that which is formed of a plurality of sheets or lamellae of one or another kinds of material, with the sheets being corrugated and arranged generally parallel to the axis of the tower in which they are installed. The sheets are corrugated and provided with holes or apertures. The holes or apertures are known to facilitate gas or vapor distribution within the packing, particularly laterally of the packing, and also to act as liquid distributing devices affecting the flow pattern of liquid moving across the sheets. The sheets are preferably corrugated, with the corrugations arranged at angles to the tower axis so that the corrugations of adjacent strips criss-cross. This latter construction makes it unnecessary to use various spacers or other supplementary devices to position the sheets with respect to one another, unless that is especially desired, since the criss-crossing ridges of the plates provide sufficient mechanical strength to maintain the plates in the desired position, especially if they are wrapped with binding material, or are spot welded or otherwise connected at their points of contact.
Early examples of this class of structured tower packing are taught in Stedman U.S. Pat. No. 2,047,444 and Huber British Patent No. 1,004,046. More recently, efforts have been made to improve the performance of this kind of packing by various sorts of surface or through-the-surface treatments. Examples of tower packing within this class having such treatments include: U.S. Pat. No. 4,296,050 to Meier; U.S. Pat. No. 4,604,247 to Chen et al., West German application No. 3,414,267.3 to Raschig.
The performance of tower packings and other vapor liquid separation devices such as trays is commonly evaluated by a parameter defined as Height Equivalent to a Theoretical Plate (H.E.T.P.), as first proposed in an article by W. A. Peters appearing in the June 1922 issue of Journal of Industrial and Engineering Chemistry. The H.E.T.P. is expressed in linear dimensions, such as, feet, inches, meters or centimeters, and the lower or smaller the H.E.T.P., the better the efficiency of the vapor-liquid contact device under consideration. H.E.T.P. is often plotted against parameters which are indicative of vapor and liquid flow rates such as F-factor (defined as V.sub.s [D.sub.v ].sup.0.5, (lbs/ft.sup.3).sup.0.5 ft/sec.) and C-Factor (defined as V.sub.s [D.sub.v /(D.sub.1 -D.sub.v)].sup.0.5, ft/sec.). Where V.sub.s =Superficial velocity, ft./sec.; D.sub.v =Vapor density lbs./ft.sup.3 ; and D.sub.1 =Liquid Density, lbs/ft.sup.3. It is generally preferred that plots of H.E.T.P. against such parameters produce curves which are as flat as can be had, over as broad a range of the flow rate parameter as possible. Such flat curves are indicative of good performance over a wide range of operating conditions, including the region near flooding at high flow rates and at very low liquid rates, where the volumetric flow rate of liquid may be so low that not enough liquid is available to wet out the entire plate surface of the packing.